To find the server reboot time, from HMC event log....

hscroot@hmc2:~> date
Mon Mar 26 04:09:01 PDT 2012


hscroot@hmc2:~> lssvcevents -t console|grep -i imme

time=03/26/2012 04:05:56,text=HSCE2121 User name hscroot: Immediate shut down executed successfully on partition SERVER1 with ID 14 on the managed system 8233-XXXX*XXXBA8P.


time=03/26/2012 03:39:49,text=HSCE2123 User name hscroot: chsysstate -r lpar -m Server-8233-XXX-SNXXXBA8P -n SERVER1 -o shutdown --immed command was executed successfully.


Note:
1)    03/26/2012 03:39:49  rebooted via HMC command
2)    03/26/2012 04:05:56  rebooted via HMC GUI